General Model Output Discussion

[Cecil Hogmanay Melchett]

Looking good this morning with a 7 day cold spell looking likely.

Ecm ens look cold out until next weekend and gfs ens remain cold right through.

[Guest North Sea Snow Convection]

If I was honest Tamara I always thought this SW would be a pain which is why I said last night the window of opportunity is small. I still think Friday holds alot of convective potential and who knows we still might see some changes between +72 & +96.

Personally I would love a 1050mb Scandi HP with -15C upper temps being blown in on a strong E,ly. However we might have to wait for this.

It seems churlish to suggest it Dave, bearing in mind the colder December we have seen, but the coldest wintry weather always has the Scandi/Siberian High working in tandem with the Greenland High. She is the missing link atm in terms of getting the less marginal air you (and I!) would like to see

Edited December 29, 2009 by North Sea Snow Convection

[Matty M]

Nice of the NAE not to update on today of all days.

[Tuxedo]

Interesting Tamara. If your right then i find it reassuring but surely GH blocking and a southerly track brings in the cold from the east in the same way as a scandi block and a southerly track.....and therefore regardless of the scenario as long as the source is the same it should be the same cold. What I am suggesting is the source is not as cold as it was a decade ago..

[nick sussex]

I still think theres one more hurdle to overcome for the uk and this appears towards the end of this weekend when that small low heads ne towards the sw and that small shortwave heads sw from the ne. There is a larger scatter in the ensembles for this time the further south and i think i prefer the ecm solution which drives much more energy into the southern arm of the jet and drops heights well into the Med, a much safer way of extending the cold at this stage.

In terms of the extended outlook we need a little help here from the eastern USA trough. Take a look at these charts from the GEM 00hrs run, note here how the USA trough edges ne'wards and remains deep this helps to stop the Greenland high from retrogressing too far westwards, this then helps downstream by edging the pattern further east with the Scandi trough better aligned as the jet digs south further east. This is a good example of how a feature upstream that perhaps is being overlooked with the current emphasis on near term events can be crucial to events in the UK.

http://www.wetterzentrale.de/pics/Rgem1441.html

http://www.wetterzentrale.de/pics/Rgem1681.html

http://www.wetterzentrale.de/pics/Rgem1921.html

Although the cold spell continued in Scotland and the far north it was the block being too far west which ultimately lead to the less cold conditions further south around Xmas so in this instance the western based negative NAO will always allow chances for less cold or mild air to head ne towards southern Britain and eventually a trough develops too far west sending southerlies northwards.

This is something to keep an eye on in future output, if you want a long cold spell and you're further south especially you want to see the models edge the block further east and go with this trend from the GEM.

Edited December 29, 2009 by nick sussex

[swilliam]

The level of unknown for the current situation is exemplified by the current GFS ensemble.

We are 24 hrs away from the maximum Northward extension of the mild air and still the GFS ensemble cannot resolve this. There is a spread from -6 to +2 for the peak T850 temperature here.

Looking further out the mean stays below -5 till 11th Jan which is pretty amazing really - almost 2 weeks of cold weather. We just need to wait for some developments on a relatively small scale or the low to the SW to produce some snow for us.

I also agree with what TEITS said earlier about the De Bilt ECM emsemble. If the mean is very low then it is bound to rise in FI - there will always be some mild options and these will bring the mean up (if the run went on then eventually it musty default to the mean). However we should keep an eye on how many of these mild solutions there are because as KOLD said there do appear to more on this run.

Edited December 29, 2009 by swilliam

[Frosty.]

Good to see the GFS 06z more in line with the GEM 00z with a N'ly flow for most of next week, the only difference being the gem kept it going for longer whereas the gfs terminates it with high pressure although it would remain cold once the high builds in with widespread frosts. Coming back to the present and near future,with winds freshening from a NE'ly direction, lots of snow showers will gather in northern and eastern scotland, northeast england and later in the southeast once that weakening front clears away from the south.

[Guest North Sea Snow Convection]

Interesting Tamara. If your right then i find it reassuring but surely GH blocking and a southerly track brings in the cold from the east in the same way as a scandi block and a southerly track.....and therefore regardless of the scenario as long as the source is the same it should be the same cold. What I am suggesting is the source is not as cold as it was a decade ago..

It is never very straightforward, as there are variations on a theme with blocking to the north of us, and it also depends on how the -NAO profile sets up in the atlantic. As I just stated, the best way to get widespread cold and snow over the whole of the BI is when both Scandinavian and Greenland High pressure are present - and they alternate which means that pressure rises to the north east as troughs sink south through Scandinavia and this pulls the winds round to the east. This high then retrogresses westwards to wards Greenland then this in turns pulls down another surge of cold air from the arctic to renew the pattern. As this moves south, then pressure rises once more to the north east. However, because the pattern is west based, we are keeping a trough over Scandinavia without the pressure rises to the north east to back the coldest air westwards over the bulk of the UK. With pressure high mainly to the NW , then the coldest air is always going to be on our eastern and northern periphery - as troughs sink southwards and cut off the cold advection from the north east, whilst at the same time, atlantic lows push in from the south west with the result that we are 'over squeezed', so to speak - but most especially the further south and west you are.

Edited December 29, 2009 by North Sea Snow Convection

[phil nw.]

The shortwave to the north was not modelled until now to develop the way it is, and therefore there was a good chance that the easterly flow coming off the back of the low pressure might be sustained for a longer period of time to bring ever colder upper air in from north east europe. Both the ECM and GEM (especially) consistently modelled this. However, it may be that the interruption is brief. There are signs that some of this colder air may still arrive after the weekend - but more output required to confirm this

HI Tamara,

I am wondering whether we need a classic Easterly though with the current modelling.

Thinking here beyond the current low and after it`s eased away.

Looking at the projected ht.comparisons ECM/GFS for 8-10 days.Here,

http://www.meteo.psu.edu/~gadomski/ECMWF_0z/hgtcomp.html

This shows the -nao still a little West and the UK under the Scandi trough.

This pattern easier to view here on the NH models.

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=168&mode=0&carte=1

http://www.meteociel.fr/modeles/ecmwf.php?ech=168&mode=1&map=1

It would seem unlikely we would get a classic easterly from this setup as the flow would generally be slack and more from the north or north East.

However by this time we would have quite cold enough air over the whole of the uk,look at these projected uppers,

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=168&mode=1&carte=1

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=168&mode=1&carte=1

As i said yesterday in this cold setup with the upper trough coming across Scandi towards us there iis always a chance of minor lows or troughs coming South in the flow.

Here an example of what could turn up next week.

http://www.meteociel.fr/modeles/gfse_cartes.php?&ech=168&mode=0

That`s apart from any convection pctn picked up from the N.sea.

It`s good to be able to discuss degrees of cold and snow for a change rather than looking for something to turn up from a mild setup, as in most winters.

[kold weather]

Im not sure I agree with some of the posts suggesting the block sinking SE in F.I. if you notice with the GFS its always in low res mode that this trend is suggested. At the moment im favouring the GEM ensemble mean which has been very consistent recently.

http://91.121.94.83/...ns-21-1-180.png

As for the E,ly being shortlived then to me its no surprise as this has been the trend for many days now. The simple reason for the E,ly is due to the S,ly tracking LP so it stands to reason this is shortlived as it transfers E.

Finally onto the ECM ensembles. Due to the mean being so low I would be utterly amazed if the mean didn't rise. However note at +240 the mean is at 0C. However I bet in a few days the mean for the +192 period is lower than currently suggested.

I see no reason why this cold spell will come to an end at the moment. The block may transfer W and bring us a W based NAO but to be honest the models are slowly backtracking from this especially the GEM.

Yeah FWIW I think that GEM mean isn't actually a bad call either however it cannot get any further west then that, ot with such an active southerly jet stream. We are likely to see an attack in the 168-240hrs range from the SW and if the block is too far west then there will be nothing to stop the low become absorbed by the PV nearby and swing NE over the UK bringing in less cold air...its very similar to what we have seen this week, which whilst good for more northern locations, its certainly not as good for the south...

I'd stil lmuch rather have a less cyclonic east based -ve NAO then a west one that could quite easily breakdown in a vigarous jet stream that we have...but a west based -ve NAO gives a decent chance of a LP attack saying that...but obviously thats a rather risky evolution...

Still odds look decent IMO for a good 10 day cold spell at least which is certainly better then recent years...IF the Greenie high does breakdown then as long as t get over the UK and not to ofar west then we should keep the cold surface conditions, if it sets up too far west then it'll be easier for the SW's to get to the UK plus the flow will probably be much more cloudy and thus less cold.

[TEITS]

It seems churlish to suggest it Dave, bearing in mind the colder December we have seen, but the coldest wintry weather always has the Scandi/Siberian High working in tandem with the Greenland High. She is the missing link atm in terms of getting the less marginal air you (and I!) would like to see

I know exactly what you mean and so far we seem to be missing one jigsaw piece. However the updated Met O forecast will put a smile on your face.

Doesn't really resemble the ECM output especially middle of next week.

http://www.metoffice.gov.uk/weather/uk/uk_forecast_weather.html

GEFS ensembles are fantastic with the mean only rising above -5C in distant F.I.

http://charts.netweather.tv/ensimages/ens.20091229/06/t850Cambridgeshire.png

[Cloud 10]

Finally onto the ECM ensembles. Due to the mean being so low I would be utterly amazed if the mean didn't rise. However note at +240 the mean is at 0C. However I bet in a few days the mean for the +192 period is lower than currently suggested.

Just a quick word about the ecm ensembles for de bilt,once the winds turn more northerly this location will "warm up" due to its proximity to the north sea.

Its a shame we can't see the 850 temp ensembles for the ecm as this would give us a better idea of the setup.

FWIW i think the 00z GEM is the most likely solution with its northerly reload.

[Duane S.]

Some very good points being made about the problems we will face if the -NAO becomes to west based.

What we need to see is a nice deep trough sitting off the Eastern seaboard to help prevent the blocking ambling too far west. Although I suspect it is going to end up going a bit too far west for comfort before swinging back round towards the east later on.

[Snowaddict]

The latest UKMO update at the moment is fantastic in my opinion..

An easterly feed of sorts is suggested into next week..

If we can get something similar to the 06Z GEFS Mean @ T+144, then I will be thrilled.

SA

[Guest North Sea Snow Convection]

HI Tamara,

I am wondering whether we need a classic Easterly though with the current modelling.

Thinking here beyond the current low and after it`s eased away.

Looking at the projected ht.comparisons ECM/GFS for 8-10 days.Here,

http://www.meteo.psu...0z/hgtcomp.html

This shows the -nao still a little West and the UK under the Scandi trough.

This pattern easier to view here on the NH models.

http://www.meteociel...&mode=0&carte=1

http://www.meteociel...68&mode=1&map=1

It would seem unlikely we would get a classic easterly from this setup as the flow would generally be slack and more from the north or north East.

However by this time we would have quite cold enough air over the whole of the uk,look at these projected uppers,

http://www.meteociel...&mode=1&carte=1

http://www.meteociel...&mode=1&carte=1

As i said yesterday in this cold setup with the upper trough coming across Scandi towards us there iis always a chance of minor lows or troughs coming South in the flow.

Here an example of what could turn up next week.

http://www.meteociel...&ech=168&mode=0

That`s apart from any convection pctn picked up from the N.sea.

It`s good to be able to discuss degrees of cold and snow for a change rather than looking for something to turn up from a mild setup, as in most winters.

Hi Phil - if we were able to get to the position that the models suggest as you illustrate above - then that is certainly a good position indeed. And yes, it is a very refreshing change to be able to discuss desgrees of cold for a change rather than looking for anything which is even the cooler side of mild!

It still relies on there being enough of a trigger to pull those colder upper temps our way. I think for north eastern parts of the UK it matters less, but for more southern, south eastern and south western parts then a rather more east based pattern is needed in order to sustain a more arctic/polar continental flow that gets colder 850's across the north sea rather than straight down it.

Edit: Dave, yes the update gives support to something like the GEM and latest GFS ouput

Edited December 29, 2009 by North Sea Snow Convection

[bluearmy]

the 6/15 dayer is pretty representative of the GEM 00z and not too far away from the ECM. there is obviously a pretty good chance of that trough heading south during next week. the ecm takes it to the sw of us, the gem to the se. at 10 days range, thats not too different. either way, increased instability + temps below freezing will mean only one thing ................ more imbyism on here !!

[kold weather]

Just a quick word about the ecm ensembles for de bilt,once the winds turn more northerly this location will "warm up" due to its proximity to the north sea.

Its a shame we can't see the 850 temp ensembles for the ecm as this would give us a better idea of the setup.

FWIW i think the 00z GEM is the most likely solution with its northerly reload.

Actually I think the GEM solution isn't that grand for longer term prospects, granted we would still have a very prolonged cold spell but wth the PV sinking south-eastwards being a very wel lagreed upon factor in the longer term the upper high should do the same thing, but obviously set the upper high up too far west then the whole lot will come down toofar west and we have a real risk that the whole lot topples totally...I'd feel much more comfortable if we can get the upper high right over the UK, gives us more room to change down the line without any milder intrusions.

FWIW the models are quite suggestive of a possible snow risk for the south/south-west between the 5-7th, its something I'd keep a very close eye on actually given the global set-up we are in with a very active southerly jet stream does suggest such risks are possible.

The longer term evolution of a ECM/GEM evolution is for eventually a deep low pressure to force the ridge ENE/NE to some degree wherever it ends up...thats why I think we cannot afford for it to sink too far south...I think thats why the ensembles are actually milder for the De blit, not the fact that they go for a northerly but rather we end up in a less cold Nw airflow that hasn't got any real bite to it and probably rather cloudy as well, as most Mid Atlantic high pressure cells tend to be in the UK...

Edited December 29, 2009 by kold weather

[Tom D]

First things first - this winter is already a lot different to any I can remember in the recent past. These charts are somewhat unprecedented! Long may the charts continue to output such eye candy I say!

With some of the charts I'm actually lost for words Another lengthy cold spell heading our way and not even January?!

Edited December 29, 2009 by Tommyd1258

[phil nw.]

Hi Phil - if we were able to get to the position that the models suggest as you illustrate above - then that is certainly a good position indeed. And yes, it is a very refreshing change to be able to discuss desgrees of cold for a change rather than looking for anything which is even the cooler side of mild!

It still relies on there being enough of a trigger to pull those colder upper temps our way. I think for north eastern parts of the UK it matters less, but for more southern, south eastern and south western parts then a rather more east based pattern is needed in order to sustain a more arctic/polar continental flow that gets colder 850's across the north sea rather than straight down it.

Edit: Dave, yes the update gives support to something like the GEM and latest GFS ouput

Based on current output the cold uppers get here from the upper flow from the Scandi trough.

I guess the difference is they seep here rather than dramaticaly from the East behind a cold front,which would be thew case from rising pressure to our N.E.

The uppers are generally forecasted to be around -5 to -8C, based on the charts i linked earlier,not really low, but plenty cold enough for snow to fall i reckon.

I guess what you may be looking for is the really bitter Easterly setup ala Jan.1987 where many locations even here got plenty of snow.

That would be wonderfull but hey this setup is not a bad second prize,if it were to verify.

Edited December 29, 2009 by phil n.warks.

[Yeti]

Interesting Tamara. If your right then i find it reassuring but surely GH blocking and a southerly track brings in the cold from the east in the same way as a scandi block and a southerly track.....and therefore regardless of the scenario as long as the source is the same it should be the same cold. What I am suggesting is the source is not as cold as it was a decade ago..

I'm not sure that's necessarily true - it just so happens that this time the cold is held back by the placement of the shortwave over Scandinavia:

http://www.wetterzentrale.de/pics/Rtavn721.png

Remember that just before Christmas sub-20 air was entrenched over here and even stretched into France. On occasion, exceptionally cold air can make it to Europe, and that still happens (although perhaps less often).

However, the Arctic is warming so in theory the probability of cold air reaching Europe ought to decrease (although not year-on-year - some years the Arctic is warmer than others obviously).

As for the "perfect" scenario for cold to reach the UK... that would have to be the Arctic high moving S like 1987:

http://www.wetterzentrale.de/pics/archive/ra/1987/Rrea00119870112.gif

http://www.wetterzentrale.de/pics/archive/ra/1987/Rrea00219870112.gif

I can't imagine any other scenario could possibly advect such cold air over the UK.

[Guest North Sea Snow Convection]

Based on current output the cold uppers get here from the upper flow from the Scandi trough.

I guess the difference is they seep here rather than dramaticaly from the East behind a cold front,which would be thew case from rising pressure to our N.E.

The uppers are generally forecasted to be around -5 to -8C, based on the charts i linked earlier,not really low, but plenty cold enough for snow to fall i reckon.

I guess what you may be looking for is the really bitter Easterly setup ala Jan.1987 where many locations even here got plenty of snow.

That would be wonderfull but hey this setup is not a bad second prize,if it were to verify.

No actually, my expectations are certainly not as high as that. Like most, just a reasonable covering of snow (rather more than 1cm to 2cms, lasting longer than 48 hrs of which is the sum total so far) and some sustained frosty cold weather with wintry sunshine and an occasional snowfall everyother day or so to keep things topped up will do perfectly fine

Latest output is very encouraging for some way ahead - much as I have already stated. And should it verify, I am sure that most of us will be very happy.

[Methuselah]

I'll make a new thread shortly, folks...In time for the 12Z bonanza!

Closing this een now...